1. Field of the Invention
The present invention generally relates to the improvement of sonar transmitting beam patterns and, more particularly, to the use of a differential transformer as a feedback sensor for generating an error signal sensed at the face of an element in an acoustical transducer array.
2. Description of the Prior Art
Each element in a sonar transmitting array is subjected to the energy of all other transmitting elements in the array. It is therefore possible that each element is vibrating at an amplitude and phase other than that which is intended. This leads to distorted transmit beam patterns. Thus, there is a need for a system which is capable of sensing and correcting the actual amplitudes and phases of the vibrations of the individual elements in the transmitting array.
In the prior art, there is known a transducer face-velocity control system for an array of underwater acoustic transducers as disclosed in U.S. Pat. No. 3,311,872 to Andrews, Jr., et al. According to that invention, the face velocity of each individual transducer is sensed either directly or indirectly (i.e., by sensing the displacement or acceleration of the face) by a second or auxiliary transducer mechanically coupled thereto. The face velocity is converted into an electrical signal and fed back as a negative feedback signal to the electrical input terminals of the individual amplifier driving the transducer whose face velocity is to be sensed. By making the product of the amplifier gain and the ratio of the feedback to input signals substantially greater than one, the face velocity of the radiating transducer is substantially independent of radiation impedance and is proportional to the driving signal, the proportionality being controlled by the feedback factor. The sensor used by Andrews, Jr., et al. is essentially a loudspeaker coil. One problem of the Andrews, Jr., et al. system is that it suffers from cross-talk.
U.S. Pat. No. 4,412,317 to Asjes et al. discloses a transducer for picking up mechanical vibrations in seismic waves. The transducer comprises two separate coils disposed in permanent magnetic fields produced by a magnet assembly. The coils and the magnetic assembly are mounted for relative movement to each other. The magnet assembly is structured and disposed relative to the two coils so as to reduce the electromagnetic coupling between the two coils to zero. One of the coils is connected to an amplifier input while the other one of the coils is included in a feedback circuit of the amplifier. When mechanical vibrations are applied to the transducer, the velocity difference between the inertial mass of the transducer and the housing thereof is reduced to zero by the current in the coil which is in the feedback circuit.
U.S. Pat. No. 3,559,050 to Mifsud discloses a velocity sensitive geophone which comprises two windings on the same coil form. The output signal produced by one winding is coupled to the other through an electrical circuit which amplifies the signal and adjusts the phase thereof. When the signal applied to the second winding is in phase with the output signal of the first winding, the device behaves as an accelerometer, but when the signals are 90 degrees out of phase, it behaves as a very low-frequency-sensitive geophone.
U.S. Pat. No. 3,208,545 to Doty et al. discloses a circuit for minimizing the phase variations occuring between a reference signal used to control a seismic vibrator and the eleastic signal transmitted by the vibrator when it is coupled to a different propagating media. In U.S. Pat. No. 4,056,163 to Wood et al., an accelerometer located on the pad of a vibratory seismic source provides a signal for comparison with the sweep signal which controls the vibrator. U.S. Pat. No. 4,286,332 to Edelmann discloses compressional wave vibrators symetrically disposed on opposite sides of a profile line of geophones. The Edelmann system includes a feedback circuit for the cancellation of compression waves. U.S. Pat. Nos. 3,718,900 to Holmes, Jr., 3,354,983 to Johnson III, and No. 3,354,983 to Erickson et al., show examples of prior art transducer structures.
What is needed is a sensor that can operate in such a manner that it is sensing the mechanical motion of a transducer element and is unaffected by either an electric or a magnetic field.